1. Field of the Invention
The present invention relates to an electric circuit board having conductive parts for electrical connection with electric component devices through electric connecting members and, more particularly, to an electric circuit board on which the conductive parts and the electric connecting members are connected through solder melted by a light beam.
2. Description of Related Art
Various types of electric circuit boards are used for electric circuits which controls electric machines. For example, as shown in FIG. 6, an electric circuit board 80 is used for a driving circuit (not shown) which drives a thermal head 90 in a facsimile machine. The thermal head 90 is connected electrically to the electric circuit board 80 through a plurality of conductive pins 95 (95a, 95b). The pins 95 have respective top ends supported by bases 93 aligned in the longitudinal direction at the lower end side of the thermal head 90 and are molded by a resin material 94. A plurality of pads 82 (82a, 82b) are formed in a copper film pattern on a base plate 81 of the circuit board 80. The bottom ends of the pins 95 are soldered onto the corresponding pads 82. The bases 93 of the pins 95 are connected electrically to a printer head driver IC (integrated circuit, not shown) mounted on the thermal head 90, while the pads 82 are connected electrically to the driving circuit (not shown) mounted on the base plate 81.
The thermal head 90 has a plurality of heater elements 91 each of which are arranged to extend longitudinally. The head driver IC, receiving driving signals from the driving circuit through the pads 82, pins 95 and bases 93, supplies electric power to the heater elements 91, which in turn heat the ink to print information on a print paper (not shown). A heat sink or heat radiating plate 92 made of metal is provided to radiate the heat of the thermal head 90 therethrough.
Soldering the bottom ends of pins 95 onto the corresponding pads 82 is attained by a soldering apparatus (not shown). The soldering apparatus feeds string solders 61 (61a, 61b) from its arm 60 onto the bottom ends of the corresponding pins 95 and projects a laser beam 96 as a laser beam onto the top ends of the string solders 61 to heat and melt the solders 61.
The laser beam 96 is set to project onto the solders 61 to cover a predetermined fixed range from the rightmost solder 61a to the leftmost solder 61b. In the event that the laser beam projection starts from a point deviated leftward from the rightmost solder 61a, the solder 61a on the rightmost pin 95a will not be heated thus failing to solder the rightmost pin 95a with the corresponding rightmost pad 82a. In the event that the laser beam projection starts from a point deviated rightward from the rightmost solder 61a, on the contrary, the laser projection will end before it reaches the leftmost solder 61b. The solder 61b on the leftmost pin 95b will not be heated thus failing to solder the leftmost pin 95b with the corresponding leftmost pad 82b. As a result, the circuit board 80 and the thermal head 90 will not be connected electrically.
It is therefore required to direct the laser beam 96 toward the right position highly accurately. As the string solder 61 is very thin, however, it is difficult to direct the laser beam 96 toward the right solder 61 in position. directing the laser beam 96 accurately to the right position necessitates a longer period of time resulting in inefficient laser-soldering operation.
To solder all the pins 95 onto the corresponding pads 82 without fail, the range of projecting laser beam 96 is expanded in actual practice so that the laser beam projection moves in a direction Fl starting from a position 83 located at the right side of the rightmost pad 82a on the base plate 81 and ending at a position 84 located at the left side of the leftmost pad 82b on the base plate 81. Thus, the rightmost and leftmost pins 95a sand 95b are soldered assuredly onto the corresponding pads 82a and 82b respectively, even when the starting position of projecting the laser beam 96 is deviated from the originally designed position.
This arrangement, however, causes a drawback in that surrounding areas 85 and 86 adjacent to the rightmost and leftmost pads 82a and 82b are likely to change quality of plate material or be burnt by the heat of the laser beam 96. Those change in quality or burning at the sides of the base plate 81 tends to cause the surface of the base plate 81 to peel off from the burnt parts. As this peeling progresses as the facsimile machine vibrates in operation, rigidity of the base plate 81 lowers at the places where the peeling has occurred. Thus, electrical connection between the rightmost pin 95a and the pad 82a is likely to be damaged, thus disabling required operation of the thermal head 90 in the facsimile machine.